The invention relates to a process and to an apparatus for changing the curvature of the cornea.
Processes and apparatus of this type are used, for example, for correcting hyperopia.
A process of the above-mentioned type and an apparatus for changing the curvature of the cornea are known, for example, from the article "Laser Thermokeratoplasty by Means of a Pulsed Holmium: YAG Laser for Hyperopic Correction" by Theo Seiler, published in Refractive and Corneal Surgery, Vol. 6, September/October 1990, Page 335, and on, or from U.S. Pat. No. 5,334,190 of the same content.
Additional processes and apparatus for the treatment of eyes are known from Patent Documents WO 89/06519, WO 90/12618, WO 91/00063, GB 2 228 344, EP 0 480 995 B1, EP 0 581 339 A2 or DE 40 04 423 A1.
Reference is explicitly made to the above-mentioned documents for explaining all details not explained in this documents.
All embodiments of processes and apparatus of this type known from these documents have in common that a pulsed laser is used which emits light in the infrared wavelength range.
Because of the use of a pulsed laser, the tissue on which the Laser beam is focussed is not uniformly heated. On the contrary, during the lumination, the tissue is heated by the energy entered by a laser pulse and will cool again because of thermal diffusion during the interval to the next laser pulse.
Therefore, because of the use of a pulsed laser, it is not possible to adjust the temperature in the tissue to be treated to a value which is as constant as possible and which--as described in U.S. Pat. No. 5,334,190 should be between 60.degree. C. and 70.degree. C.
On the contrary, in the case of the known processes and apparatuses,--as recognized according to the invention--temperature peaks may occur which are above 100.degree. C. At these temperatures, the tissue will rapidly relax so that the endeavored treatment success of correcting a defective vision by the selective shrinking of the collagenic tissue is not achieved or is achieved only insufficiently.
It is an object of the invention to provide a process and an apparatus for changing the curvature of the cornea by means of which the defective vision of an eye and particularly hyperopia can be reliably corrected by shrinking the collagenic tissue.
According to the invention, for the selective shrinking of the collagenic tissue, the light is applied continuously at such a power that, although a temperature is reached which results in irreversible coagulations of the collagenic tissue, the temperature within the coagulated region will rise only to values at which a relaxation of the tissue does not yet occur. In this case, the light source is preferably a laser diode with a power of between 150 mW and 500 mW, preferably approximately 180 to 200 mW. In the case of such a laser diode, the lumination time for each laser spot is between 3 and 10 seconds.
Furthermore, it is preferred for the temperature in the coagulated region to be approximately 50.degree. C. to 65.degree. C., since, at these temperatures, a coagulation will already occur but no relaxation of the tissue. In individual cases, higher temperatures may also be used. However, it was recognized according to the invention that a relaxation may already occur starting at approximately 70.degree. C.
By means of the approach according to the invention in which the energy required for the coagulation is applied continuously at a comparatively low power and not pulsed at a high power, a gentle treatment of the cornea is obtained in each case.
The entering of energy at a comparatively low power selected according to the invention also permits the selection of the luminous power of the light source such that, in the region of the focus cone, an approximately spherical coagulation region is formed by means of which the corneal endothelium is not impaired. This form of the coagulated region leads to a better long-term stability compared to the known processes with a pulsed entering of energy.
Furthermore, because of the gentle form of the entering of energy selected according to the invention, it is possible to place the tip of the focus cone in deeper layers of the stroma than in the case of the known methods. As a result, a particularly effective shrinking of the collagen tissue is obtained. Naturally, it is required that the depth of the focus is still sufficiently above the endothelium so the latter is not damaged or is damaged only minimally. Typical values for the depth of the focus, that is, the distance of the tip of the focus cone from the surface of the cornea, are approximately 460 .mu.m to maximally approximately 800 .mu.m.
In the case of an apparatus for implementing the process according to the invention, a continuously operating laser diode is preferably used as a light source whose wavelength amounts to 1.9 .mu.m. The output power of the laser diode typically amounts to between 150 mW and 500 Mw, preferably between 180 and 200 mW. A focussing lens system focusses the light of the laser diode into the cornea. An optical fiber is preferably arranged between the laser diode and the lens system.
In the case of a laser diode, it is possible to vary the wavelength by changing the diode temperature, for example, bar means of a Peltier element. As a result, it is possible to change the depth of the focus by a variation of the wavelength of the laser diode. In this case, the diameter of the laser spot on the surface of the cornea remains virtually the same.
As an alternative or in addition, for changing the depth of the focus, the distance can be changed between the light exit surface of the optical fiber and the lens system. In this case, the diameter of the laser spot on the surface of the cornea will also change. Typically, the diameter of the laser spot is between 0.3 mm and 0.6 mm.
In a preferred embodiment of the invention, the lens system is arranged in a handpiece. In order to always ensure defined conditions, it is advantageous that the front of the lens system can be placed on the cornea because no arbitrary distance variations can occur then.
A planoconvex lens whose front is the convex surface can be used as the lens system. The lens system can be made of suitable materials which are transmitting for the wavelength of the used laser diode, such as sapphire.
Without limiting the general idea of the invention, the invention will be described in the following by means of embodiments with respect to the drawing. Explicit reference is made to the drawing with respect to the disclosure of all details of the invention which are not explained in the text.